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DNase Test PrincipleThe deoxyribonuclease (DNase) test detects the degradation of DNA by bacterial species that produce DNase. The DNase test may be performed on plate media and is available in some commercial tests, e.g., QuadFERM+. When DNase is produced by organisms in the QuadFERM+ test, an acidic endproduct is formed and the pH indicator changes from red (alkaline) to yellow (acid) e.g., Figure 1. Figure 1. Example of a Rapid DNase Test for Neisseria and Related Species. Optimum specimen: Pure culture (18-24 h.) of gram-negative, oxidase-positive, catalase-positive diplococci grown on nonselective (chocolate or equivalent) medium isolated either from selective (Modified Thayer-Martin; MTM) medium or nonselective (chocolate or equivalent) medium. Unacceptable specimens:
Factors that compromise test results:
Stability of specimen:
Medium/ReagentsDNase tests may be performed on plate media which incorporate indicators. However, most media for DNase tests do not support the growth of fastidious Neisseria species. QuadFERM+: Commercial, an acid detection test, includes a DNase test. Store in accordance with manufacturer's directions. Quality Control/Test ProcedureQuality control of the DNase test is performed concurrently with QC of the acid detection component of the QuadFERM+ test. QC strains:
Procedure:
Table 1. DNase Production Patterns of Quality Control Strains of N. gonorrhoeae and M. catarrhalis.
Quality Control Schedule:
Table 2. DNase Reactions of Neisseria and Related Species. The control cupule containing no carbohydrate remains red (no acid has been produced). Cupules with contents that are more yellow than the contents in the control cupule are recorded as positive.
Problems & SolutionsIncubation of the strips in a incubator with an enhanced carbon dioxide atmosphere may cause false-positive reactions because carbon dioxide will dissolve in the media and form carbonic acid, a weak dibasic acid which, in turn, will cause an acid reaction in the medium. Incubate tests only in incubators without supplemental carbon dioxide. Strips must be warmed to room temperature before use. The test relies on preformed enzymes. Inoculation of cold test strips may reduce the activity of the enzymes Saline containing preservatives, buffers, or bacteriostatic agents should NOT be used in preparing bacterial suspensions; these may denature the preformed enzymes that react with the substrates. Do not make bacterial suspensions in water; enzymes may be denatured. Use pure cultures only. Mixed cultures may include organisms that produce acid from carbohydrates different from those of the test neisserial strain. Use bacterial suspensions immediately (within 15 min after preparation from cultures that have been held at room temperature no longer than 30 min). Enzymes may denature if suspensions are not used immediately. If many strains are to be tested, inoculate each test strip immediately after preparation of the suspension rather than preparing all suspensions and then inoculating tests; this inoculation strategy will also minimize the possibility of mislabeling a test strip. Limitations of TestThe DNase is used only as a supplemental test to aid in the identification of Neisseria and related species which have been characterized in other tests such as acid detection tests. The DNase test distinguishes M. catarrhalis from all other gram-negative diplococci of human origin. Only N. ovis, a species isolated from sheep and cattle, produces DNase and fails to produce detectable acid from carbohydrates. It should be noted that strains of N. ovis have occasionally been isolated from humans. The colonies of N. ovis are translucent and butyrous compared with those of M. catarrhalis which are opaque and usually friable. The DNase test may not be used to differentiate between other Neisseria and related species. Results, Interpretation, & ReportingThe DNase is used only as a supplemental test to aid in the identification of Neisseria and related species which have been characterized in other tests such as acid detection tests. BibliographyBovre K. 1984. Family VIII. Neisseriaceae Prevot, p. 288-309. In Krieg NR (ed.). Manual of Systematic Bacteriology, vol. 1. The Williams & Wilkins co., Baltimore. Knapp JS. Historical perspectives and identification of Neisseria and related species. Clin Microbiol Rev 1988;1:415-431. Vedros NA. 1984. Genus I. Neisseria Trevisan 1885, 105AL, p. 290-296. In Krieg NR (ed.). Bergey's Manual of Systematic Bacteriology, vol. 1. The Williams & Wilkins Co., Baltimore. Page last modified: October 17, 2008 Page last reviewed: October 24, 2008 Content Source: Division of STD Prevention, National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention |
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